What is STEC and why is it a risk in sprouted seeds?

4 June 2025 | Suzanne Jordan, Section Lead Molecular Microbiology and Methods

Having recently been accredited with an extension to scope for STEC detection in sprouted seeds (TES-MB-225), we explore the wider issue of STEC, why it is such a risk in sprouted seeds and how STEC testing can help support food businesses.

Why are STEC a cause for concern

As many of you may already be aware, the incidence of severe foodborne illness as a result of pathogenic contamination is on the rise, particularly from Shiga toxin-producing Escherichia coli (STEC).

Additionally, there has been a two-fold increase in non-O157 STEC infection notifications in the UK since 2018, and a STEC O145 outbreak in salad in the UK reported in 2024.

So, what do we mean when we talk about STEC, and why is it so important to combat?

What are STEC?

STEC refers to a group of E. coli that contain genes coding for the “Shiga toxin” (stx) which is also known as Verocytotoxin.

Although E. coli O157:H7 is the most well-known strain, many hundreds of STEC serogroups exist and all have the potential to cause human illness, with those able to attach to the gut wall causing the most severe illness.

Subsequently, its growth represents a serious health threat, as not only is STEC highly infectious, but it only requires just a few bacteria to cause illness with symptoms that can vary from mild to severe.

The problem with STEC, is that if it is consumed within a food, it begins to multiply when it reaches the gut. Those that can attach to the gut wall will do so potentially causing the first primary symptom of severe STEC infection: haemorrhagic colitis or severe bloody diarrhoea. If that isn’t bad enough, the Shiga toxin produced by this organism can in some cases enter the bloodstream and potentially cause kidney damage, leading to haemolytic uremic syndrome. Such severe symptoms can result in hospitalisation and, in the worst-case scenario, death.

Why sprouted seeds are a risk

In recent years, sprouted seeds in salads have become increasingly popular among health-conscious consumers.

Given the very nature of sprouted seeds, there are a number of factors that can contribute to STEC contamination including:

The sprouting process

Firstly, during sprouting the seed coat breaks down releasing nutrients which could enable microbial growth.

The warm, humid conditions used for sprouting can promote bacterial growth, enabling low levels of pathogens to rapidly increase in numbers.

The many potential contamination sources

Another challenge is that seeds could be exposed to STEC contamination at various stages during production and post-harvest handling including storage and distribution.

How they are consumed

Sprouted seeds are often added to salads, meaning they are consumed either raw or with minimal cooking. The lack of processing to reduce microbial numbers heightens the risk of consuming pathogenic organisms if they are present.

How can controlling the risks of STEC

It is crucial to reduce the contamination risks by following good agricultural and handling practices (including thorough cleaning and storage), and cooking sprouts thoroughly before consumption.

STEC detection can be used to monitor the effectiveness of control measures implemented to reduce the risk of product contamination.

The role of STEC testing and analysis

Currently, in the EU, manufacturers of sprouted seeds have a legal requirement to test seeds destined for sprouting or sprouted seed irrigation water for STEC serogroups O157, O26, O103, O111, O145 and O104:H4. Any samples that are found to be positive are not to be placed on the market.

Through the use of molecular methods like PCR, it is possible to detect STEC contamination in food products before they reach consumers. The assays detect the stx genes and eae genes as well as the presence of one of the top six serogroups.

However, STEC analysis does not provide a simple yes/no answer, as often there are many answers making data interpretation problematic.

Consequently, one of the key challenges faced by food businesses is in the interpretation of results to inform food safety decisions. Consider the following questions:

What happens if one of the top serogroups is detected, but it contains no stx genes?

Similarly, what should you do, if the organism is not a STEC, but is it a risk to consumers?

These are just some of the interpretational difficulties faced by those doing STEC testing and demonstrates the need to seek expert food microbiological assistance to help in decision making.

How Campden BRI can help you

Fortunately, we have the right experience and expertise to help you.

Firstly, our Shiga toxin-producing E. coli (STEC) testing service is accredited to the ISO/IEC 17025:2005 standard by UKAS, for the detection of STEC O26, O103, O111, O145, O157, O45 and O121. And just recently, we have been granted the extension to scope for STEC sprouted seeds using the (TES-MB-225) method.

Campden BRI is also one of very few facilities in the UK to have the category three laboratories required for handling STEC, which requires an enhanced level of containment because STEC are highly pathogenic.

Our UKAS accredited testing is also available with both standard and rapid turnaround times.

UKAS accreditation is crucial for us and our clients, as it demonstrates our technical competence and the reliability of our testing and analytical services, which is essential in an industry where product safety, quality and compliance are non-negotiable.

Campden BRI have every intention of expanding our STEC accreditation beyond sprouted seeds into other known high-risk areas, so watch this space for more information. We can also test samples outside the UKAS scope if that is of interest, along with a host of support services to support your due diligence, such as risk assessments and training.

Why not take advantage of a FREE 20-minute consultation with one of our experts so we can understand more about your particular challenges? Just fill in the short online form and a member of the team will be back in touch.

About Suzanne Jordan

Suzanne has worked here at Campden BRI since 2005, following nine years of PhD and postdoctoral research experience in food microbiology and molecular biology of food microorganisms.

Suzanne is the lead for third party microbiological method validation studies for AOAC, MicroVal and NordVal, is a Retailer-approved Method Review Co-ordinator, and is involved in several research and contract projects for developing and evaluating new methodology. Alongside this, she is an industrial PhD supervisor for a project on the of fine-tuning dietary fibre to target gut microbiota accessibility.

During her career to date she has participated in multidisciplinary research projects involving European partners, developed expertise in a range of molecular techniques, and presented her research at an international level and in peer-reviewed journals.